读某年世界主要产茶国比较表，回答下列各题。

在极坐标系中，圆C过极点，且圆心的极坐标是$\left(a,\frac{\pi }{2}\right)\left(a>0\right)$，则圆C的极坐标方程是（  ）

根据上下文提示补全对话。

A: What's on the farm?

B: ____ ____ a lake and some trees.

A: Is the farm ____?

B: Yes, it's very big.

A: ____ ____ an elephant on the farm?

B: Yes, there is. It's very strong.

A: Is there a house?

B: No, there ____.